A fuel cell has an anode and a cathode which are separated by an electrolyte that transports the ions but does not conduct the electrons. The cathode-electrolyte-anode assembly is commonly referred to as a PEN (Positive electrode-Electrolyte-Negative electrode). The electrolyte consists of a metal oxide in solid, non-porous form, for example a zirconium oxide, which is capable of transporting oxygen anions. The anode, which is porous, is the place where a gaseous fuel, generally hydrogen, or even an organic fuel (methanol, light fuel, natural gas) with the hydrogen being extracted therefrom by reforming, undergoes an oxidation reaction. The oxygen of the air undergoes a reduction reaction at the cathode, which is porous. The reaction is continuous by virtue of the continuous supply of fuel gases and oxidizing gases, produces two electrons per hydrogen molecule and delivers a potential of around 0.6 to 1.2 V, as well as heat. It is necessary to associate several SOFCs in series in order to obtain a higher output voltage. In the planar SOFC concept, a “stack” is formed, each unit of which comprises one or more ceramic and/or metal plates having a thickness of from a few tenths of a millimeter to a few millimeters, integrating all the electrochemically active components of the PEN, and an interconnecting plate which is sometimes referred to as a dipolar plate.
In particular, the invention relates to an SOFC PEN with a cathode, said cathode comprising a porous cathode layer and an active cathode layer, with an anode, said anode comprising an active anode layer and a porous anode support layer, said anode support layer constituting the mechanical support of the PEN, and with at least one non-porous electrolyte layer, said electrolyte layer being placed between said active anode and cathode layers.
The document “Status of the Sulzer Hexis solid oxide fuel cell (SOFC) system development”, R. Diethelm et al., Third European Solid Oxide Fuel Cell Forum, P. Stevensed., Nantes, June 1998, p. 87-93, describes an SOFC developed by the company Sulzer Hexis (CH). The fuel gases and oxidizing gases are supplied to the electrodes by an interconnecting plate consisting of a chromium-rich metal alloy disk (94% Cr, 5% Fe, 1% Y2O3), pierced by a central hole and structured by machining on its two faces, the latter alternately having openings toward the outside and toward the central hole. In a stack of this type, the fuel circulates in the central tube and is then diffused toward the anode face of each cell by virtue of the openings left by the structure of the interconnecting plate. The fuel cannot penetrate toward the cathode side since the interconnecting plate covers the entire inner edge of the cathode over a width of a few millimeters. The air is injected from the outside through holes which are made in the interconnecting plates and vertically aligned, which makes it possible to guide the air through U-shaped profiles that are pressed against the stack. The air passes through an internal cavity of the interconnecting plate, which makes it possible to heat it by means of the heat given off by the cells. The air then passes over the structured face of the interconnecting plate, opposite the cathode. On the cathode side of the interconnecting plate, a thin ceramic layer is deposited by VPS (Vacuum Plasma Spraying) so as to prevent the evaporation of chromium in the form of CrO3. The external diameter of the interconnecting plates of the PENs is about 120 mm and that of the inner hole is 22 mm. A stack comprises about 70 cells and has a height of about 50 cm; the electrical power produced is 1 kW under 40 V. The interconnecting plates of this type of stack are complicated to produce and are expensive. The power and electrical voltage delivered by this device are a function respectively of the surface area and the number of cells, and hence of the surface area and number of interconnecting plates, the cost of which forms an obstacle to the profitability of the electrical energy produced by this type of device.
Documents JP 04 169071 and JP 04 079163 describe an interconnecting device that can be arranged between two PENs, this device differing from that of the Sulzer cell in that it is made in three parts: a planar central plate bears on each side a layer made of electrode materials in which channels are made for the circulation of the gases. The additional electrical resistance due to the contact between this layer and the electrode itself which lies opposite it is high, although it may be reduced by an additional interface made of conductive material.
The document JP 03 134964 also describes an interconnecting device consisting of three ceramic layers, one of which bears channels for the circulation of gas. Moreover, the PEN is borne by a plate-substrate, which on the side opposite the PEN has channels for the circulation of the other gas.
U.S. Pat. No. 5,256,499 (Allied Signal Aerospace) describes an SOFC stack consisting of a stack of ceramic plates which are pierced with several holes, these holes being arranged near the edges of the plates; the arrangement of the holes opposite one other forms lateral tubes for the supply and evacuation of the fuel and oxidizing gases, the active elements of the PENs being arranged in the central part. Each cell consists of a plate forming the electrolyte, surrounded on each side by at least one plate forming respectively the anode and the cathode, as well as of two plates pierced with holes having the dimensions of the anode and cathode and the same thickness as these two electrodes, surrounding the latter so as to form a tube section. This PEN is sandwiched between two planar interconnecting plates. Each electrode is itself formed either of a plate bearing channels or bumps or of the assembly of a planar plate and a wavy plate. The interconnecting plates of this device are simpler and less expensive than the plates of the Sulzer device mentioned above, but each cell requires two additional elements surrounding the electrodes so as to form the tubes. These elements, which are almost completely hollow, are fragile and it is difficult to seal between the electrodes and these elements. The structure forming the channels is sintered on the electrolyte, which means that it is not possible to compensate the planarity defects of the cell. Moreover, in this device, the plate constituting the electrolyte forms the mechanical support of the PEN. It must therefore be relatively thick and consequently has a relatively high ohmic resistance. In order to increase the efficiency of an SOFC, the ohmic resistance has to be reduced as far as possible by using an electrolyte of small thickness, something which is not possible with structures where the electrolyte forms the support.
In order to reduce the ohmic resistance of an SOFC, the document WO 00/69008 proposes the use of a relatively thick porous anode as the mechanical support of the PEN and to deposit an electrolyte in a thin layer (10 to 40 μm) and also a relatively fine counter-electrode on this anode support. However, this PEN requires interconnecting plates with a complex structure comprising the supply and evacuation tubes for the gases, and are thus quite thick, in order to form a stack. This structure is therefore disadvantageous on account of its thickness and the cost of the interconnecting plates.
Patent application WO 01/67534 describes an anode consisting of a multitude of discrete ceramic columns between which a gas can circulate, said columns being located between a fine layer of electrolyte and a metal interconnecting plate which is likewise fine. On the cathode side, the electrolyte is likewise separated from the interconnecting plate by a structure formed of a multitude of discrete columns allowing gas to pass between them. The structures consisting of the columns are produced by stamping the individual columns into a strip of raw ceramic and fixing these columns to a sheet of paper, which allows them to be handled. The sheet burns and disappears the first time the stack is used after assembly. This device makes it possible to use planar interconnecting plates which are therefore inexpensive. However, the column structure is complicated to produce, and handling of the components of the cell during assembly is delicate. Finally, it requires an additional sealed system for the supply and evacuation of the gases.
The document JP 08 078040 also describes a system of discrete ceramic columns which are stuck on each side of a planar PEN, providing the electrical connection to the interconnecting plates and allowing the gases to pass. This device has the advantages and drawbacks mentioned above in respect of the document WO 01/67534.
The document JP 06 068885 also describes a system of columns, in an arrangement similar to that of the previous document mentioned. The electrolyte plates and the interconnecting plates constitute the mechanical supports of this system, the electrodes being very fine electrodes which are printed onto each face of the electrolyte plate, which in turn must therefore be thick, thereby increasing the ohmic resistance.
Patent application WO 01/41239 also describes a system of channels formed by a multitude of discrete columns which allow gas to pass between them. The structures formed by the columns may be produced by locally depositing, over a thickness of 0.05 to 0.4 mm, materials that constitute electrodes on the two faces of a planar interconnecting plate, using a printing method. The columns thus form the electrodes. The interconnecting plate and the electrolyte plate each have at least one pair of holes in their central zones, each of the holes being surrounded alternately, on each face of the interconnecting plate, by a seal. The holes of the interconnecting plate and of the electrolyte plate come into alignment for the supply of gases, the latter flowing radially toward the edges of the plates between the columns. The interconnecting plate bearing the column electrodes is inexpensive to produce. However, handling of the electrolyte plate (0.2 to 0.4 mm thick) during stacking is delicate. In this system, like in the one described in WO 01/67534, the surface area of the electrodes is the total front surface area of the columns, that is to say only a fraction of the surface area of the plates. The ohmic resistance of the PEN is therefore greater than that of a PEN of the same composition having its electrodes in contact with the entire surface area of the electrolyte.
The document WO 01/41239 also proposes making systems of channels by producing them, mechanically or chemically, in the surfaces of the interconnecting plate or electrodes. This variant is expensive to implement, as in the case of the Sulzer interconnecting plates described above.
The aim of the present invention is to propose an SOFC PEN which makes it possible to produce a stack that does not have the drawbacks of the devices of the prior art. The invention is aimed in particular at producing stacks that can use, to interconnect the SOFCs, simple and inexpensive fine metal plates. It is also aimed at producing SOFCs, the ohmic resistance of which is as low as possible. It is also aimed at limiting the size in terms of thickness of an SOFC. It is also aimed at increasing the electrical power available per unit of surface area. It is finally aimed at producing a PEN together with its interconnection system, which is easy to manufacture and easy to handle during building of the stack.